The braking-related comfort and safety systems of motor vehicles, such as, for example, antilock braking systems, driving stability control systems, distance control systems, or rollover prevention systems, are controlled with increasing sensitivity in response to continuously increasing customer requirements. As a prerequisite for correspondingly sensitive control, however, it must also be possible to control the pump motor of the hydraulic braking system with corresponding precision. A particularly sensitive control is usually achieved by means of pulse-width modulated current signals, wherein the current signals are preferably modulated with a frequency of up to 18 kHz. Likewise, it is also possible, however, to modulate the current signals with frequencies greater than 18 kHz. In general, the greater the frequency of the modulation, the greater the sensitivity with which the pump motor is operated. It is usually also necessary that the particular angular position of the motor or of the pump be known as accurately as possible.
In this context, DE 10 2009 006 891 A1, which is incorporated by reference discloses an electric motor for a hydraulic pump in a braking control system. The electric motor consists of a rotor, which is connected to a motor shaft, and a stator having a plurality of brushes, wherein the rotor provided with turns comprises a commutator having a plurality of commutator bars. In order to permit precise detection of the angle-of-rotation position of the motor shaft, one of the commutator bars is lengthened compared to the remaining commutator bars. In addition, one of the brushes has an axial offset in order to contact the lengthened commutator bar. A signal, which is characteristic of the angle-of-rotation position, can therefore be sensed via the offset brush. Since the actual rotational speed of the motor can therefore be detected independently of the torque to be applied, the speed can always be corrected to the desired setpoint value via the duty factor of the pulse-width modulated control.
DE 10 2008 018 818 A1, which is incorporated by reference describes an electric motor control method having load torque adaptation. In this case, an electric motor is controlled by a current source, which controls in a pulsed manner. Depending on the present load, a certain motor speed results for a certain pump flow or a certain duty factor of the control current. The load during one motor revolution is not constant, however, since this is an angle-dependent load, namely the hydraulic pump of a vehicle braking system. A ripple signal is read out from the voltage potential present at the motor during one revolution and is evaluated for periodicity. It is therefore possible to set a constant rotational speed in the case of periodically occurring events by means of an angular-position dependent adaptation of the duty factor of the control pulses, which contributes to noise reduction and therefore increases driving comfort.